Cyclins are key components of the core cell cycle machinery. Cyclins bind and activate their associated cyclin dependent kinases (CDKs). Very strong evidence indicates that in addition to their roles in driving cell cycle progression, D-type (D1, D2, and D3) and E-type (E1, E2) cyclins are critically required for apoptosis induced by virtually all stimuli, including ionizing radiation. We hypothesize that cyclin-CDK complexes represent excellent targets for pharmacological intervention to prevent cell death induced by the ionizing radiation. In the proposed work we will first take advantage of mice lacking cyclin D1, D2, D3, E1, or E2, that we generated, as well as mice lacking catalytic partners of D-cyclins (CDK4 or CDK6) or E-cyclins (CDK2), made by our collaborators. These mice offer us a unique tool to test the requirement for particular cyclins and CDKs in apoptosis of various compartments, in particular of the hematopoietic cells. In Aim 1, we will test the response of mice lacking individual cyclins or CDKs to ionizing radiation. Once cell cycle proteins required for radiation-induced apoptosis are identified in Aim 1, it is critically important to determine whether an acute shutdown of a given protein after the irradiation took place would protect against cell death. We will address this in Aim 2, using inducible conditional knockout strains of D- or E-cyclins. In Aim 3, we will test several available CDK inhibitors for their ability to protect mice against the effects of radiation. We will combine these chemicals with pifithin-alpha, an agent that transiently blocks p53 action, and which was already shown to offer radioprotection. The Specific Aims are: (1) To determine whether ablation of particular cyclins or CDKs protects animals from radiation-induced apoptosis; (2) To determine whether an acute shutdown of particular cyclins after radiation took place protects mice from radiation-induced apoptosis; (3) To determine whether blocking of cyclin-CDK function by administration of CDK inhibitors protects against radiation-induced apoptosis. Our work will verify whether cyclin-CDK complexes represent good targets to prevent radiation-induced apoptosis. The ultimate goal of this proposal is to identify chemical compounds - within the five years time-frame of this grant - that protect against radiation-induced apoptosis. Several chemical inhibitors with specificities for different cyclin-CDK complexes are already available and are being used for cancer trials. These agents could be used immediately to protect humans from the effects of radiation.